Objectives were to analyze the role of smoking, drinking and their synergistic effect in the occurrence of potentially malignant oral disorders (PMOD). We examined three groups: 50 lung cancer patients, 50 patients with liver cirrhosis and 50 patients with clear medical history. Scores were developed for drinking, smoking, drinking and smoking and PMOD. All four scores were the lowest in the control group. The lung cancer group showed the highest Smoking, Alcohol & Smoking and Lesions score, while the liver cirrhosis group had the Alcohol score the highest. Compared to the control group, lung cancer group is more likely to develop a PMOD than the liver cirrhosis group (OR=12.31/ OR=6.71). Statistical significance between the groups was found in the Lesions score (χ2=15.34 ; p=0.001). The lung cancer and liver cirrhosis patients represent a high risk group for PMOD. Lung cancer and liver cirrhosis patients have never, to our knowledge, been categorized as high risk patients for PMOD. After diagnosed, lung cancer and liver cirrhosis patients should have a routine oral cavity examination, as they present a high risk group for PMOD and oral cancer.